Unrestricted GPTs are a well-studied class of GPT. The rebit
[1]}, for example, is a GPT with a disc-shaped
state space. The state space can be equivalently modeled (see Section
REF ) by the subset of real density
matrices of a qubit. Rebits are convenient low-dimensional building
blocks for a toy model of quantum theory, giving rise to many characteristic
features such as superposition, entanglement and non-locality [2]}, [1]}, [4]}.
